In underground mining operations, it is common to sink into the earth shafts which can extend one thousand or more feet in depth in the location of the stratum or layer of the particular ore or other material such as coal to be extracted and then in the course of such extraction to develop tunnels penetrating more or less laterally according to the thickness and direction of the ore stratum which can extend for additional thousands of feet. The extraction ordinarily proceeds by means of the so-called "room and pillar" method in which the ore is removed in localized areas called rooms while leaving intervening areas unmined so as to constitute pillars which aid in supporting the roof over the mine regions in conjunction with other supporting expedients such as roof bolts, bracing and the like.
Two methods of ore extraction are currently in use at least in the mining of coal which is the special concern of this invention--the so-called conventional method and the continuous mining method. In the conventional method, a deep horizontal slot is cut in an appropriate location along a section of the face of the ore seam then being worked, usually at the bottom although sometimes at the top, the face above the slot is drilled for the reception of explosives which are then detonated so as to break quantities of coal loose from the seam. The coal thus dislodged is then collected and removed from the mine by shuttle cars moving through one or more tunnels. In the continuous method, a continuous mining machine cuts away the ore from the face of the seam by means of a cutting head carried at its front end, and collects and transports the ore rearwardly thereof for loading on the shuttle cars for removal as before. In either case and particularly in the instance of continuous mining, substantial quantities of dust are generated notwithstanding the practice of maintaining the cutting instrumentalities under a continuous spray of water, and this airborne dust presents a hazard to the miners from the standpoint of both breathing and risk of explosion.
Moreover, certain deposits of coal have entrained therein large quantities of methane generated during the original formation of the coal in eons past, and as the coal is extracted, the methane escapes and tends to collect along the working face of the mine. The amount of methane released varies from mine to mine with some mines being more highly subject to this phenomenon and being known as "gassy" mines. Methane, of course, is an explosive gas particularly when combined with the proper portions of air, and the majority of the mining disasters through the years can be attributed to explosions caused by the accidental ignition, for instance, by a casually produced spark, of this methane gas.
While mining operators and companies have always been more or less concerned in minimizing the content of dust and methane in the atmosphere within the working environment of the mine, the enactment into law of the Mine Safety Act of 1969 has imposed particularly stringent regulations in order to protect the health and safety of the working miners. For instance, the permissible limit on the quantity of dust is about 2 mg/cu.m, while the amount of methane gas is continuously monitored so as to detect in advance any build-up well before the danger level is approached. For instance, the working face may be inspected with a methane detector every 1/4-1/3 hour or so and, in addition, the continuous mining machine itself will frequently incorporate an automatic methane sensing instrument which will automatically discontinue the supply or power to the machine in the event a significant change in methane content occurs.
In order to reduce the hazards of the ore dust and ignitable gas, mining operators long ago developed techniques for ventilating the mine so as to induce reasonably strong currents of fresh air therein. Bearing in mind that as the extraction of the coal proceeds over long periods of time, an involved system of shafts, tunnels and corridors can evolve which can take on nearly labyrinthian proportions, it will be immediately apparent that if the required currents of fresh air were to be created throughout the entire open area of such a mine, the cost would be completely prohibitive. In order to bring this cost within tolerable limits, mining operators and companies adopted the practice of suspending flexible cloth curtains, called brattice cloth, in appropriate locations so as to confine the flow of the air as closely as possible to the region immediately contiguous to the working face of the coal seam by closing of those rooms in which the ore extraction had been concluded or temporarily interrupted as well as side tunnels or corridors not currently in use. These curtains can be hung along lines which can be curved as necessary to act as partitions for confining and directing the air flow as desired and in that instance are referred to as line brattice. Also, it is common to provide a temporary closure across the tunnel or shaft which serves for ingress and egress to the working area of the mine as well as for the transportation of the extracted coal out of the mine, and these partitions usually are referred to as check or fly curtains.